The invention relates generally to methods for the synthesis of substituted fullerene compounds and in particular to methods for the synthesis of carboxylated buckminsterfullerene compounds having utility as antioxidants and neuroprotective compounds. Even more specifically, the invention relates to methods for the synthesis of bis, tris and higher adducts of C60 having very high regioselectivity and product yield.
Multiply-substituted fullerenes are useful for discovery of new pharmaceuticals. Murphy et al., U.S. Pat. No. 6,162,926, disclose multiply substituted fullerenes and describe their use in combinatorial libraries. The compounds have pharmaceutical, materials science and other utilities. FIGS. 1 and 2 are schematic representations of the bisadducts and trisadducts, respectively, disclosed in Murphy et al.
Using malonate groups (E1xe2x80x94CH2xe2x80x94E2) and the so-called Hirsch-Bingle reaction, fullerene compounds can be synthesized with groups substituted at many different sites. Wilson et al., Organic Chemistry of Fullerenes; Fullerenes: Chemistry, Physics and Technology, Kadish, K. M. and Ruoff, R. S., eds., John Wiley and Sons, New York, 2000, pp. 91-176.
Bingel, U.S. Pat. No. 5,739,376, describes the following reaction: 
where E1 and E2 are COOH, COOR or other radicals and n is 1-10. Several of these compounds, e.g. the so-called carboxylated buckminsterfullerenes have become potentially useful as pharmaceutical candidates for the protection of neurotoxic injury. Choi and Dugan et al., PCT/EP97/02679. FIG. 3 depicts a trisadduct (C3) obtained by Choi.
The large scale synthesis of C3 is difficult since a multitude of isomers are produced and the preparation requires HPLC separation of the desired isomer for use as a therapeutic. One way to control the substitution on the C60 is by the so-called tether-directed addition process. Investigators have tried linking a multitude of chemically reactive groups together so that they react with the C60 only at one site. A complete survey of these attempts is found in Wilson, et al.
It is known that water soluble fullerene hexaacids like those shown in FIGS. 4 and 5 are effective antioxidants and have neuroprotective properties. It is desirable to produce larger quantities of these compounds for clinical studies.
The usual synthetic precursors for the compounds of FIGS. 4 and 5 are the hexaesters. These can be made by stepwise reaction of C60 with diethylbromomalonate and intermediate purification by flash chromatography. The reaction has been described in Bingel., Chem. Ber. 1993, 126, 1957. Due to the stepwise synthesis and the tedious chromatographic purifications, the yield of trisadducts is low. This reaction is thus unsuitable for large scale production.
Diederich et al. have developed a method for the one-step production of e,e,e- and trans-3, trans-3, trans-3 trisadducts from C60 using a cyclotriveratrylene tether. G. Rapenne et al., Chem. Commun. 1999, 1121. Although this reaction leads to a clean formulation of trisadducts, the overall yield is still quite low, e.g. 11% trans-3, trans-3, trans-3- and 9% e,e,e-isomer, and the tether system itself is only accessible in a multi-step synthesis.
While some success has been achieved using these methods to link one or more reactive groups to a single fullerene, more effective processes are required if multiply substituted fullerenes are to be used in drug discovery.
It is a primary object of the invention to provide methods for producing water soluble fullerene polyacids.
It is a further object of the invention to provide methods for producing water-soluble fullerene hexaacids for use as neuroprotectant therapeutic compounds.
The invention is broadly in a method for synthesizing compounds of the formula 
where C60 is a C60 fullerene.
The method comprises the steps of forming a macrocyclic malonate compound of the formula 
where each Z is the same or different and is a straight-chain or branched-chain aliphatic radical having from 1-30 carbon atoms which may be unsubstituted or monosubstituted or polysubstituted by identical or different substitutents, in which radicals up to every third CH2 unit can be replaced by O or NR where R is alkyl having 1-20 carbon atoms or a chain containing unsubstituted or substituted aryl or other cyclic groups and n is an integer from 2 to 10; reacting said macrocyclic malonate compound with C60 to form a macrocycle adduct of the formula 
where the Z radicals are linked together to form said macrocycle adduct; and hydrolyzing said macrocycle adduct to form a compound of the formula 